Polyolefins are useful in any number of everyday articles. However, one drawback to many polyolefins, especially polypropylene homopolymers and copolymers, is the relatively high glass transition temperature. This characteristic makes these polyolefins brittle, especially at low temperatures. Given that there is a desire to make articles that can withstand a broad range of temperatures, there is a need to provide a polyolefin that can maintain its desirable characteristics such as high temperature performance, etc., while improving upon the impact strength and toughness. In particular, it would be advantageous to provide a propylene polymer possessing improved toughness without sacrificing its toughness and other desirable properties.
Addition of a plasticizer or other substance to a polyolefin is one option to improve such properties as impact strength and toughness. There are some disclosures towards such an end, such as U.S. Pat. Nos. 4,960,820; 4,132,698; 3,201,364; WO 02/31044; WO 01/18109 A1; and EP 0 300 689 A2. These disclosures are directed towards polyolefins and elastomers blended with functionalized plasticizers. The functionalized plasticizers are materials such as mineral oils which contain aromatic groups, and high (greater than −20° C.) pour point compounds. Use of these compounds typically does not preserve the transparency of the polyolefin, and impact strength is often not improved.
Other background references include EP 0 448 259 A, EP 1 028 145 A, U.S. Pat. Nos. 4,073,782, and 3,415,925.
What is needed is a plasticized polyolefin with improved softness, better flexibility (lower flexural modulus), a depressed glass transition temperature, and improved impact strength (improved Gardner impact) at low temperatures (below 0° C.), while not influencing the melting temperature of the polyolefin, the polyolefin crystallization rate, its clarity, and with minimal migration of the plasticizer to the surface of articles made therefrom. Further, there is a need for a plasticized polypropylene that can be used in such applications as food containers and toys.
It would be particularly desirable to plasticize polyolefins by using a simple, non-reactive compound such as a paraffin. However, it has been taught that aliphatic or paraffinic compounds would impair the properties of polyolefins, and was thus not recommended. (See, e.g., CHEMICAL ADDITIVES FOR PLASTICS INDUSTRY 107–116 (Radian Corp., Noyes Data Corporation, NJ 1987); WO 01/18109 A1). The inventors of the present invention have found, surprisingly, that paraffinic compounds are useful plasticizers for polyolefins. The present invention is directed towards such a plasticized polyolefin.